In the prior art it is common to store the multiple riser sections from which the subsea riser string is composed in a riser storage of the vessel.
Commonly a riser section comprises a riser pipe and in many known embodiments additionally one or more satellite or peripheral pipes on the outside of and along the riser pipe. The satellite pipes are e.g. used as fluid lines to a BOP or other subsea equipment, e.g. choke lines, kill lines, hydraulic lines, booster lines, injection lines (e.g. for glycol), etc. Each riser section comprises a connector fitting arrangement at each end thereof. For example the connector fitting arrangement includes a flange having bolt holes, with riser sections being joined by interconnecting flanges by means of bolts and nuts. A satellite pipe may have an individual connector fitting, e.g. a bayonet fitting, or be designed to fit sealingly into the satellite pipe of an adjoining riser section without direct axial securing of the satellite pipes. In many practical embodiments a riser section is provided with one or more buoyancy and/or thermal insulation members, e.g. of foam material, but so-called bare joints are also employed.
Riser sections come in different lengths. Commonly riser sections have lengths between 50 ft. (15.24 meters) and 90 ft. (27.43 meters). A very common length for riser sections is 75 ft. (22.86 meters).
Riser sections are commonly heavy; far heavier than other tubulars used in the offshore drilling industry. For example a single 75 ft. subsea riser section may weigh between 20 and 25 tonnes, which is incomparable to the weight of an equally long drill pipe. Therefore riser handling is subject to different considerations than drill pipe handling, mainly in view of their size (diameter) and weight.
For example WO2009/102196 discloses a mono-hull vessel having a hull and a riser storage hold within the hull. In the riser storage hull riser sections are stacked in their horizontal orientation. A gantry crane is provided to raise and lower the riser sections out of and into the storage hold and to place each individual riser section onto a riser catwalk machine or to pick up a riser section from the catwalk machine. The leading end of the riser section is in practice connected to a riser string lifting tool which connects the riser section to a riser string handling capacity hoisting device of the vessel. By raising the lifting tool and operation of the catwalk machine the riser section is brought into a vertical orientation, or upended, in line with a firing line along which the riser string is suspended into the sea. The already launched portion of the riser string is then temporarily held by a riser string hanger, often referred to as a riser spider, of the vessel. The new riser section is then held in alignment above the launched riser string and the connector fitting arrangements are interconnected to join the new riser section to the riser string. Then the riser string is released by the riser string hanger and lowered over the length of the newly attached section. The riser string is then suspended again from the riser string hanger and the process of joining a new riser section is repeated.
It has been found that this known process to assembly a riser string is time-consuming. In particular a great deal of effort has to be made to properly make up the connections between the connector fitting arrangements of the riser sections. In particular in view of desired or required testing of each connection that has been made up the known process is undesirably slow.